Add active series metric to ingester stats page

[songjiayang]

Is your feature request related to a problem? Please describe.
Now, the cortex ingester stats page has some series yet, is it possible to add the cortex_ingester_active_series metric?

[yeya24]

@songjiayang This would be a nice feature to have. Would you like to contribute?

[songjiayang]

very happy to.